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Old 17th August 2012, 10:46 AM   #21
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Make the cap big enough
can you give an example?
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Old 17th August 2012, 10:48 AM   #22
AndrewT is offline AndrewT  Scotland
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At least half an octave below the input filter is big enough to avoid that voltage across the capacitor.

Let's say you set the input filter to 50ms. Then set the NFB filter to >75ms.
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Old 17th August 2012, 11:38 AM   #23
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At least half an octave below the input filter is big enough to avoid that voltage across the capacitor.
Let's say you set the input filter to 50ms. Then set the NFB filter to >75ms.
I've lost you.
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Old 17th August 2012, 12:02 PM   #24
AndrewT is offline AndrewT  Scotland
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I set my input filter to ~90ms. That requires a combination of a 9k1 resistor and a 10uF capacitor.
That input filter makes sure that all frequencies significantly below 1.7Hz do not develop any significant voltage across the feedback capacitor.
At frequencies around 1.7Hz the feedback capacitor will develop a little voltage. But if the NFB capacitor is set to a value that ensures that frequencies around 1.7Hz do not develop a significant voltage across the capacitor, then that capacitor will not add any distortion to the signal being processed. By adjusting the impedance of the capacitor one can also adjust or eliminate the voltage across the cap. To achieve this desirable set of circumstances all one has to do is select a capacitor whose impedance at 1.7Hz is low in comparison to the NFB resistances (that set the amp gain).
You do this by selecting a RC time constant for the NFB capacitor that is at least half an octave below the input filter. i.e. Rc>140ms. This would require the NFB resistor to be 1k0 and the capacitor to be 150uF (150ms > 140ms) or any other RC combination that gives >140ms.
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Old 19th August 2012, 10:31 PM   #25
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Default Midrange, Bass, Midbass, Treble. . . hidden 4 band eq in there

Quote:
Originally Posted by zakman35 View Post
Can you give a place to start? Nichicon Muse for instance would be an option?
Nichicon ES, Panasonic FC and other high efficiency caps could be on the shortlist of things to try. You can do a search for favorites to see what other people use. The idea of optimal differs per each application, so I'd try 5 favorites to see what you like. Price does not indicate performance, but datasheets can be helpful. Polar caps work fine in split rail amplifiers.

Midrange, forward, recessed or level: For Non-Inverting LM3886 amplifiers, it might be good to choose amplifier board power caps with large enough capacitance to help support a level midrange frequency response prior to attempting the selection of any signal caps. The Panasonic FC 1500uF that AudioSector uses for power caps of the amplifier board, might be a good, first place to start. Good power must come first.

Bass harmonic balance and clarity: The size of the NFB cap versus the size of the Input cap affects the H1/H2 bass harmonic balance whereby too small Nfb cap makes warm/boomy/muddy bass, too large Nfb cap makes rumble bass monotony and somewhere in-between would have a balance with neither problem. The muddy bass problem is most commonplace and that muddy problem happens when the input cap passes more bass than the NFB cap. So, the simple solution for clearer bass would be to have the Input cap smaller and/or the NFB cap larger.
AndrewT probably explained this better than I could, so long as your calculator is up for it.

Treble Bypass cap: For leveling out the treble, those inexpensive little green polyester dip caps from Tracon/Xicon are very easy to use in parallel to an electrolytic cap. Applicable size range are from approximately 1nF to 150nF and it takes some experimentation to see which is most attractive in combination with your electrolytic cap for getting the clarity that comes from leveling the treble response.
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Last edited by danielwritesbac; 19th August 2012 at 10:35 PM.
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Old 20th August 2012, 09:50 AM   #26
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Thank you all for your helpful & detailed explanation.

In order to summarize this, I have to do the following modifications.
  • I have 96.000uf of capacitance at PSU +/-35V ----- I guess no problem with midrange.
  • Changing the input filter from 1k & 1.5n (Fc=106hz, 1.500msec) to 9k1 & 10uf,(Fc=1.7hz, 91msec) -----will this affect the input sensitivity of the amp?
  • Replace the feedback resistor of 1k with a 687 ohm, in order to increase the gain from 20,6 to 29,5 approx.
  • Replace the NFB capacitor with a 220uF = 151msec. ---- Is it Ok to try some non-polar MKT caps I already have in hand?
  • What about the tweeter amps, do I have to follow the above example or it is ok to just increase the NFB capacitance from 10 to 22uf.
  • Adding bypass caps on tweeter channel.

One more question, why the schematic indicates non-polar caps at NFB?
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Old 20th August 2012, 10:29 AM   #27
AndrewT is offline AndrewT  Scotland
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if the NFB cap has significant impedance at audio frequencies then it has an effect on the audio signal.
If you use a capacitor in the NFB that sees an actual audio signal then you must use a good capacitor. Some form of plastic film cap.
BUT THIS IS NOT the way to operate a Power Amplifier. It is not an active filter.
The NFB cap should not have a significant impedance in any part of the audio band.
If you make the NFB cap too small then you are asking the amp to operate as a filter. DON'T.
Use a cap that is big enough such that the passive filters at the input ensure that a significant voltage can NEVER build up across the NFB cap. If there is no voltage across the NFB cap, then there can never be any distortion due to the capacitor.
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Old 20th August 2012, 10:50 AM   #28
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Quote:
Originally Posted by zakman35 View Post
Thank you all for your helpful & detailed explanation.

In order to summarize this, I have to do the following modifications.
  • I have 96.000uf of capacitance at PSU +/-35V ----- I guess no problem with midrange.
  • Changing the input filter from 1k & 1.5n (Fc=106hz, 1.500msec) to 9k1 & 10uf,(Fc=1.7hz, 91msec) -----will this affect the input sensitivity of the amp?
  • Replace the feedback resistor of 1k with a 687 ohm, in order to increase the gain from 20,6 to 29,5 approx.
  • Replace the NFB capacitor with a 220uF = 151msec. ---- Is it Ok to try some non-polar MKT caps I already have in hand?
  • What about the tweeter amps, do I have to follow the above example or it is ok to just increase the NFB capacitance from 10 to 22uf.
  • Adding bypass caps on tweeter channel.

One more question, why the schematic indicates non-polar caps at NFB?
Very quickly...

1. 96000uf capacitance. You could reduce that by a factor of... oooh lets not go there... 30

2. 1K and 1n5 to 9K1 and 10uf will kill most of the audio. Also the 9K1 will reduce input sensitivity as it forms a divider with the 19.6k. It's reducing the level by nearly half.

3. Feedback resistor change is fine to alter the basic gain.

4. If you have non polar caps of that value then its fine to try. Electroylitics aren't all bad though

5. As I think I mentioned earlier increasing the 10uf feedback caps to 22uf just preserves the same low frequency roll off point due to the new value of fedback resistor.

6. Adding bypass caps.... not sure quite what and where you mean.

Why non polarised shown. Maybe just the way the "artist" drew it. I do that sometimes. If he really meant to use a non polarised its because he believes it may be sonically better. That's up to you to decide and evaluate. I would have no issue with electros when used correctly and correctly specified.
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Old 20th August 2012, 07:42 PM   #29
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Quote:
Originally Posted by zakman35 View Post
. . .[*]I have 96,000uf of capacitance at PSU +/-35V ----- I guess no problem with midrange.. . .
I wasn't talking about power board, but that is more than sufficient.
However. . .

I was actually talking about the amplifier board, such as your schematic shows.
Click the image to open in full size.
The photo is from Linkwitz Lab. Do you see what's missing? The amp schematic shows the total capacitance on the amplifier board is a pair of 100nF??? There's a problem.

Let's fix.
Click the image to open in full size.
The photo is from Audiosector. See the two big blue cylinders on the amplifier board? You want them for your amplifier board.

In document AN1192, National Semiconductor promotes 470uF caps for the Power Caps Of The Amplifier Board; however, larger caps (like 1500uF) will have a more laid back presentation that is usually more useful.
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Old 21st August 2012, 10:14 AM   #30
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Ok thanks for the notification.
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